This application is continuation-in-part application of U.S. patent application Ser. No. 09/281,678, now abandoned, filed on Mar. 30, 1999; and, is based on patent application Ser. Nos. 217518/1998 filed on Jul. 31, 1998 in Japan and 030772/1999 filed on Feb. 9, 1999 in Japan, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a water-repellent coating and a coating film to be formed by using such a water-repellent coating, especially relates to a coating made of a mixture of a tetrafluoroethylene resin and a silicone resin and a coating film thereof.
2. Description of the Related Art
A water-repellent coating and a coating film thereof have been used in many products that require the characteristics of water repellency, anti-snow-coating, and anti-ice-coating. A representative example of the conventional water-repellent coating is a fluororesin coating as one having the characteristics of excellent weather resistance and stain resistance for use in the fields of building construction, automobile industry, and so on. The conventional fluororesin coating is of single component system using a copolymer of fluoroolefin with a variety of hydrocarbons. A coating film formed by such a coating has a contact angle of about 80xc2x0 (i.e., the angle which a drop of water forms with a surface of the coating film), resulting in limited water repellency.
In addition, there is a two-component based coating that comprises tetrafluoroethylene as a major constituent thereof which is mixed with a fluororesin (e.g., vinylidene fluoride resin). This kind of the coating provides a coating film with high water repellency as shown by a contact angle of about 150xc2x0 with water, so that it is nearing practical use. In this case, however, the two-components based coating requires the expensive fluororesin powder in high volume. In spite of its excellent initial water repellency, there are the problems that the characteristics of water repellency, anti-snow-coating, and anti-ice-coating of the coating film can be depressed by subjecting or immersing it in water for a long time.
FIG. 1 is a schematic cross sectional view of the conventional coating film formed using fluororesin binder and fluororesin powder. The coating film comprises a binder 53 applied on a substrate 51. The binder 53 includes fluororesin powder 52. As shown in the figure, some particles of the fluororesin powder 52 are partially exposed out of the surface of the binder 53. In this case, furthermore, we can observe a reduction in the water repellency under the condition of immersing the coating film in water for a long time. It can be caused as a result of water retention in a space 54 between the fluororesin powder 52 and the binder 53 because water can generally enter into such a space 54.
In addition, unevenness occurs as air bubbles exist in the coating film. Depending on the size of the air bubble, several troubles can be caused, for example the surface of the substrate 51 may be exposed out of the coating film and a pin-hole may be formed on the coating film. Therefore, these troubles result in the decline in the qualities of coating film and make an ugly appearance of the coating film.
FIG. 2 is a schematic cross sectional view of a coating film having the air bubbles. A silicone resin binder 62 comprising an abundance of water-repellent particles 61 is applied on a substrate 64. Some particles are insufficiently immersed in the binder 62, as shown in the figure, so that a part of each of them is protruded through the surface of the coating film. If air bubbles are in the silicone resin binder 62, imperfections 63 in the coating film can occurred by the presence of air bubbles when the coating is dip-coated or brush-coated on the substrate 64 or when the applied coating is dried. At least a part of the substrate surface is exposed to the outside, resulting in reduction in the quality of coating film and the loss of the beautiful appearance thereof. In addition, the solids content in the coating is another cause of such troubles. If the solids content is low, the viscosity of the coating is decreased and thus imperfections 65 in the coating film are generated as a result of dropping the solids in the coating film by its own weight during the step of drying.
For keeping the coating film in high water repellency and high anti-ice-coating, the coating film should be prepared using a high purity PTFE resin powder. In addition, a coating should be performed by means of dip-coating or brush-coating when the coating is applied on a complex structure or at a place where fine spray of the coating should not be released into the air. However, the PTFE resin powder has its own surface with low free energies, so that it is difficult to conform the solvent and the resin. As a result, a lot of air bubbles are incorporated into the coating when the PTFE resin powder is dispersed therein. Therefore, air bubbles cause imperfections in the coating film when the coating is dip-coated or brush-coated on the substrate. Depending on viscosity properties of the coating, furthermore, troubles such as imperfections can be caused as a result of dropping the solids in the coating film by its own weight.
The present invention represents a substantial improvement over the prior art from the perspectives of avoiding the deterioration of water repellency with time. Such deterioration is caused by any alterations to qualities of the binder and the fluororesin powder in a surface of the coating film. The alteration can be caused by the penetration of water into crevices formed between the fluororesin powder and the binder as a result of the difference between their surface free energies and poor wettability.
Therefore, it is an object of the present invention to provide a coating and a coating film prepared by applying such a novel coating on a substrate. The coating is made of a mixture of tetrafluoroethylene resin and silicone resin. The coating has the properties of high water repellency, excellent anti-ice-coating, excellent anti-snow-coating, and so on, in spite of without comprising a large amount of fluororesin powder. In addition, the coating film keeps the water repellency regardless of whether the coating film is dipped into water for a long time.
In a first aspect of the present invention, there is provided a water-repellent coating, comprising:
tetrafluoroethylene resin powder;
silicone resin binder; and
organic solvent, wherein
the tetrafluoroethylene resin powder has a peak absorbance of infrared absorption spectrum approximately at 1,800 cmxe2x88x921 that reflects the presence of a carbonyl group as a terminal group and a peak absorbance approximately at 500 cmxe2x88x921 that reflects Cxe2x80x94F bond, where the ratio of the peak absorbance at 1,800 cmxe2x88x921 to the peak absorbance at 500 cmxe2x88x921 is from 0.0001 to 0.05 exclusive.
Here, the silicone binder may be one selected from a group consisting of polyorganosiloxane, fluorinated polyorganosiloxane, and a mixture thereof.
The organic solvent may be one selected from a group consisting of: alcohol solvents, aromatic solvents, aliphatic solvents, and mixtures thereof.
The organic solvent may be one selected from a group consisting of: fluoro solvents, hydrocarbon solvents, ketone solvents, and mixtures of at least two of these solvents.
The ratio of change in specific gravity of the whole may be less than 10%.
The solid content consisting of the tetrafluoroethylene resin powder and the silicone resin binder may constitute 20 to 40% by weight and the organic solvent may constitute 60 to 80% by weight, on the basis of a total weight of the water-repellent coating.
A de-foaming treatment may be performed.
The solids content consisting of the tetrafluoroethylene resin powder and the silicone resin binder may constitute 20 to 50% by weight and the organic solvent may constitute 50 to 80% by weight, on the basis of a total weight of the water-repellent coating.
In a second aspect of the present invention, there is provided a water-repellent coating film to be coated on a substrate, comprising a coating as described herein.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.